Posted
by
timothy
on Friday January 31, 2014 @11:25PM
from the In-hollow-halls-beneath-the-fells dept.

the_newsbeagle writes "Chinese engineers love their superlatives: Biggest dam, fastest train, etc etc. Now they've constructed the deepest underground dark matter detector beneath a mountain in Sichuan province. Such dark matter seekers have to be buried deep to shield them from cosmic rays, because that radiation would be picked up by the detector and could be confused for radiation generated by dark matter. Other dark matter detectors are similarly subterranean: LUX, in the United States, is at the bottom of an abandoned mine in South Dakota, and a European effort called XENON lies below the Gran Sasso mountain. The Chinese researchers hope their PandaX detector will finally reveal the much-hypothesized, never-seen dark matter particles known as WIMPs."

Actually, XENON isn't a European project, it's an international collaboration with leadership in the United States and members in Europe and China. The device is in Europe, but that's sort of incidental. Here's the membership: XENON-100 [columbia.edu]

We're talking about that one decimal place before the decimal point: current observations indicate more than five times as much dark matter as known matter. It's not just a little bit of round-off error in how much dust there is between stars, but apparently most of the stuff in the universe (at least, before you get to "dark energy," which is "twice as big" again, but far more poorly understood).

The gap is large than that, as the 5:1 ratio given there is from things like the CMB, and not direct observation of matter. Direct observation of matter only finds about a quarter or less of the matter that the such a model predicts, so saying there is 5 times as much dark matter as matter already assumes there is a huge chunk of normal matter we haven't yet seen. No one is assuming we are omniscient and have seen all there is to be seen, although there are some quantitative upper bounds on specific kinds

Given the vast distance between stars, saying that most of the matter in the universe is dust between stars seems reasonable to me.Isn't that like saying there is more dust floating in the air than there is in concentrated dust bunnies in your house?Another example would be saying there is alot more water vapor in the air that what you see concentrated in the clouds.I didn't do the calculation but it doesn't seem unreasonable to me to think there is more matter between us and the neared star thanthere is co

We can make estimates of the amount of dust out there based on the light we see from distant stars. Where there is dust, it will scatter light passing through it (and modify the spectrum). There are lots of open questions about how much and what kind of dust is out there --- this isn't a "solved" problem --- however, best estimates plus known uncertainties don't put this within range of explaining dark matter. So, we still need dark matter to "make the math work out."

Given the vast distance between stars, saying that most of the matter in the universe is dust between stars seems reasonable to me.

If there were that much dust, there would be a LOT more light scattering from stars and other effects.

Why do we need to introduce dark matter to get the math to work out?

It's not just like we have some random idea that there's a large amount of mass out there. The gravitational effects suggest it is concentrated in certain areas, certain parts of galaxies, etc. If the matter were simply dust or something concentrated enough according to those gravitational patterns observed, it would have major observable effects.

Because of Big Bang nucleosynthesis [wikipedia.org]. Long story short, we can look at how the universe formed from evidence left behind in the cosmic microwave background radiation, and see that it is impossible for normal baryonic matter to explain dark matter. That, and the Bullet Cluster [wikipedia.org] makes little sense without dark matter.

Sounds like what you're talking about are compact halo objects [wikipedia.org], e.g. brown dwarfs, black holes, etc. The broad consensus is that there is nowhere near enough of this material to explain the high velocity of stars in the outer parts of the galactic disk/s. Also we have observations such as the bullet cluster [wikipedia.org] which are very difficult to explain without proposing invisible gravitating material.

So perhaps you can explain this to me: The bullet cluster is special because the observed gravitational lensing has largely broken free from the observed luminous matter, continuing on as though the collision had not occurred. But why would dark matter behave like that? As I understand it, in a galactic collision it is exceeding rare for more than trace amount of matter to actually impact each other, mostly it's all just gravitational interactions, and thus DM should be affected similarly. So why would

The reason that the dark matter separates is the same reason that it follows a different distribution from visible matter in other galaxies (a diffuse blob instead of a galactic plane): whatever the stuff is, it's very weakly interacting with everything else (including other dark matter). Normal matter experiences "drag" as you push one cloud of it through another --- from the particles interacting and bouncing off of each other (or, at least, more gently being pushed by electromagnetic radiation from other

In very broad brush strokes, the impression I get is that normal matter is 'sticky' in that it interacts with photons. Things get ionized, they clump up. Because dark matter is taken to not interact at all with photons or electromagnetic fields in general it tends to just pass right through. If you're talking about a diffuse material that *only* interacts gravitationally then what you'd expect is for it to crash on through while the baryonic gas gets waylaid by intervening material.

We just had a story about "people doing something on a computer" last week. Not much has changed about the existence of computers since last week. No need to post articles bothering with the trivial details between one computer system and another --- it's all the same, once you've seen one. Only alert me when something new happens in the world.

What collider experiments do look for is "missing mass" --- if the amount of stuff coming out doesn't add up to the stuff going in, minus known detector inefficiencies (indicating some new "invisible" particle being produced). So, people are looking for dark matter production --- anything that results in mass/energy being converted to unknown/undetectable forms --- in a systematic manner. You may not specifically be looking for "spacetime foam," but anything that doesn't mimic "ordinary" particles in firing

Who assumes matter-antimatter collisions are "nullfiications"? They don't produce "nothing," but spit out particles with the "combined" properties of the inputs --- i.e, back-to-back photons carrying the energy and momentum of the initial particle/anti-particle pair. Also, if DM is some localized warpage in space-time, why does it appear gravitationally bound to visible masses (i.e. accumulating in galactic halos)? Space-time wiggles (like gravity waves) don't behave that way in known theories. unless you'r

A marble mountain? Here I've been under that impression that both granite and marble had a detectable amount of radioactivity of their own, so even given 20 miles of the stuff, there would still be a background count contaminating the data.. Can someone fact check me on that?

The biggest worry for a dark matter experiment is neutrons. Alpha radiation and beta radiation are charged particles, and are stopped by a modest amount of material because the electromagnetic force is strong. Gamma radiation is photons, but because there are so many charged nuclei and electrons in matter, a gamma ray will eventually interact in enough shielding. Typically that's something like a a meter of lead. But neutrons aren't charged, so they don't experience the electromagnetic force, and so they ca

Just about every kind of rock has background radiation of its own, which must be dealt with (some more than others). However, radiation from rocks is typically easier to deal with than cosmic rays from space --- it's lower energy stuff that can be blocked by a few extra layers of extra lead/copper shielding (carefully screened for even lower radioactivity), instead of energetic particles that go through hundreds of meters of material unhindered. You have to worry about things like radon (radioactive gas) seeping out of the rocks and getting into the equipment; but, these are known effects to watch out for deal with by proper ventilation/sealing.

I know that the detector tank in the bottom of Homestake is lead shielded, but that lead is very old, no newly mined lead in it. It had to be at least 100 years old to even be considered for recycling into that shielding. I used to live in Rapid City in the '60's, even have a wife I still miss buried there, but in those years, Homestake, 50 miles away in Lead, SD was an actively producing gold mine. And environmental disaster as it struggled to remain profitable, it eventually had to close, and I am glad that another use has been found for its extended underground.

The Lead/Deadwood area tried to survive on tourists, but I imagine much of that allure has faded after the state raided and closed the Pink Lady in the '70's, the countries oldest continuously operated whorehouse. The girls were clean, checked daily to keep them that way, and they contributed 5 to 7 million a year to the local charities. When they had the liquidation sale, somebody wanting a piece of history had to bid $50,000 just to get the front door. End of an era as it had been there, a fully functioning, locally respected member of the community for over 140 years. I felt a little sad at the passing of a legend.

Generally speaking, the deeper the mine is the less likely it is to have to deal with cosmogenically activated radiation sources or direct cosmic radiation. Same reason that these kinds of experiments often use "ancient lead" [aspera-eu.org] that has been buried under the seas for thousands of years: the deeper you go. Stuff that has been underground for thousands or millions of years is vastly less likely to have been made radiative from the sun or other cosmic sources.

Same reason that these kinds of experiments often use "ancient lead" [aspera-eu.org] that has been buried under the seas for thousands of years: Stuff that has been underground for thousands or millions of years is vastly less likely to have been made radiative from the sun or other cosmic sources.

Newly-mined lead is not radioactive due to the sun or cosmogenic sources, but because of isotopes of lead which are produced from decays of uranium and thorium (which already existed from the supernova remnants that formed our current solar system) --- see here [wikipedia.org]. So, you don't want lead that you've just mined from millions of years deep underground --- that stuff is still pretty hot. What you want is stuff that was mined by the Romans two millenia ago, separated from the underground crud full of uranium/thor

That's a far better explanation than mine, thanks. My wet ram, at nearly 80, doesn't always recall the scale of the age, just that it was old and they paid a high premium for it because it was old & well "cooled".

...a cosmology news story so a bunch of Slashdot armchair physicists without a lick of physics education past high school can tell the science community they've been doing it wrong for the last 70 years.

If people here were coming up with alternate theories that account for all the observations that we presently see that have continually point physicists to the conclusion that there's some substantial non-baryonic mystery component of the universe, then that would be fine. But most of the oh-so insightful wisdom being shed seems to come from people who haven't even bothered to brush up on the wikipedia articles on the topic.

While it's certainly unscientific to shut out alternate theories based solely on t